1. Field of the Invention
The present invention relates generally to a focus control method and an optical disc recording/reproducing apparatus and more specifically, to acquisition of a more accurate focus bias signal and particularly to improvement of reproducing characteristic in high density recording operation by recording a test pattern data for focus servo signal determination on a predetermined track and also recording the other pattern data on a neighboring track and then determining a focus servo signal by considering amount of crosstalk of the other pattern data when the test pattern data is reproduced.
2. Description of the Related Art
In a recording/reproducing apparatus for reproducing an optical disc such as a magneto-optical disc, data is reproduced while focus adjustment of optical pickup for the optical disc is conducted. In this case, in view of realizing the optimal reproducing characteristic, determining process of the optimum focus servo signal is performed to carry out the optimum focus servo control immediately before the data is reproduced.
Therefore, the particular pattern data recorded on the particular data area has been reproduced while a focus bias value is being changed. Just focus is defined when the amplitude of the reproduced output becomes largest. On this timing, the focus servo signal in such a focus bias value is considered as the optimum focus servo signal in the optical disc.
Meanwhile, when an optical disc for high density recording is used, the track pitch becomes narrow and therefore crosstalk of the signal from the neighboring tracks must be considered. However, in the related art, the optimum focus servo signal is determined not considering such crosstalk at all. That is, the particular data is recorded on the particular track as the focus servo data, but any data is not recorded on the neighboring track of the particular track.
When the recording density becomes high, amount of crosstalk from the neighboring track is also changed by defocusing, because the shape of beam spot is deformed by the astigmatism of the optical pickup system as will be explained later.
When crosstalk from the neighboring track is generated even under the defocusing condition, amplitude of the reproduced signal becomes large in some cases. Thus, it is impossible to simply determine that the just focus is obtained when the amplitude level becomes largest. Accordingly, in the related art, it is impossible in current to accurately determine the focus servo signal to realize the just focus condition.
The present invention is therefore intended to solve the problems of the related art explained above and it is an object of the present invention to provide a focus control method and an optical disc recording/reproducing apparatus, which can determine the optimum focus servo signal with considering crosstalk from the neighboring track.
According to a first aspect of the invention, we provide a focus control method comprising of recording test pattern data on a predetermined track of an optical disc and the other pattern data on a neighboring track of the predetermined track, reproducing the test pattern data with varying focus bias, and obtaining the focus servo signal providing the just focusing condition from a jitter amount obtained when the test pattern data is reproduced.
In this aspect of the invention, when the test pattern data is reproduced, the pattern data of the type different from the test pattern data is also recorded on the neighboring track. Therefore, the data of the neighboring track is also reproduced simultaneously. Particularly, when defocusing is occurring, amount of crosstalk also increases because shape of the beam spot is distorted.
When amount of crosstalk increases as explained above, the reproducing level becomes high in this case, but jitter element also increases. Therefore, such a mistake that the defocusing condition is erroneously judged as the just focusing condition can be eliminated by making determination with reference to the amount of jitter. That is, the focus servo signal accurately resulting in the just focusing condition can be obtained.
According to a second aspect of the invention, we provide an optical disc recording/reproducing apparatus. Preferably, the apparatus comprises a recording device for recording test pattern data on a predetermined track of the tracks provided in the test pattern area of an optical disc and the other data on a neighboring track of the predetermined track. Further, the apparatus suitably comprises a reproducing device for reproducing the test pattern data from the predetermined track with varying focus bias value, a PLL circuit for receiving the reproduced data, and a jitter amount calculating device for calculating amount of jitter corresponding to the focus bias value when a phase error signal obtained from the PLL circuit is supplied. Additionally, the apparatus desirably comprises a focus servo controlling device for determining the optimum focus bias signal based on the calculated jitter amount, compensating for a focus error signal with the focus bias signal, and also outputting the focus servo signal on the basis of the compensated focus error signal.
In this aspect of the present invention, amount of jitter of the reproduced signal is measured while the focus bias is varied. The reproduced signal also includes crosstalk. Here, the minimum amount of jitter is detected. When jitter is minimum, just focusing is defined. A focus bias value of this just focusing condition is defined as the optimum focus bias value. Thus, the focus servo signal obtained by adding the focus error signal to such optimum focus bias value is defined as the optimum focus servo signal. Since an error rate can be improved by reproducing the signal with the focus servo signal pre-adjusted as explained above, the reproducing characteristic of optical disc can be improved.
The above and other objects and features of this invention will appear more fully hereinafter from a consideration of the following description taken in connection with the accompanying drawings wherein an example is the illustrated by way of example.